Strong anharmonic and quantum effects in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>P</mml:mi><mml:mi>m</mml:mi><mml:mover accent="true"><mml:mn>3</mml:mn><mml:mo>¯</mml:mo></mml:mover><mml:mi>n</mml:mi></mml:mrow></mml:math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>AlH</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:math> under high pressure: A first-principles study
Pugeng Hou, Francesco Belli, Raffaello Bianco, Ion Errea
Abstract
Motivated by the absence of experimental superconductivity in the metallic $\mathit{Pm}\overline{3}\mathit{n}$ phase of ${\mathrm{AlH}}_{3}$ despite the predictions, we reanalyze its vibrational and superconducting properties at pressures $P\ensuremath{\ge}99$ GPa making use of first-principles techniques. In our calculations based on the self-consistent harmonic approximation method that treats anharmonicity beyond perturbation theory, we predict a strong anharmonic correction to the phonon spectra and demonstrate that the superconducting critical temperatures predicted in previous calculations based on the harmonic approximation are strongly suppressed by anharmonicity. The electron-phonon coupling concentrates on the lowest-energy hydrogen-character optical modes at the X point of the Brillouin zone. As a consequence of the strong anharmonic enhancement of their frequency, the electron-phonon coupling is suppressed by at least 30%. The suppression in $\ensuremath{\lambda}$ makes ${T}_{c}$ smaller than 4.2 K above 120 GPa, which is well consistent with the experimental evidence. Our results underline that metal hydrides with hydrogen atoms in interstitial sites are subject to huge anharmonic effects.